This invention relates to a design of an aircraft wing and is concerned with reducing drag of an aircraft wing. More particularly, this invention is concerned with high speed, turbofan powered airline aircraft, having a low aspect ratio tapered wing. The invention is directed to a wing combination for drag reduction, and also to aircraft including such a wing combination, and also to various aircraft types designed within a group of performance parameter limits, and to a method of reducing the drag of an existing aircraft.
From the very beginning of the power of flight, the design of aircraft structures has required the comprise between numerous conflicting requirements. Unlike land or water-based craft, an aircraft has to generate sufficient lift to support its own weight, and consequently minimizing weight has always been a significant design criteria for aircraft.
A corollary to this is that the design of the main wing must provide the necessary lift. The lift provided by any wing section will depend upon the section chosen and air speed. The wing should be capable of providing sufficient lift both during take-off and at cruise and other conditions. At steady cruising conditions, the thrust provided by an aircraft""s engines balances the drag on the aircraft.
A significant element of drag can be induced drag. This in turn is largely dependent upon the wing profile. It has long been known that a high aspect ratio wing, i.e. a wing which is relatively long and slender, gives much reduced induced drag. However, a long, slender wing imposes severe structural loads. In effect, the lifting forces provide large cantilevered loads and large bending moments, which reach a maximum at the wing roots, at least for a monoplane design. A slender wing shape compounds this problem by providing a reduced thickness to the wing, reducing the bending moments that the wing can carry. Designing a wing that is sufficiently strong to withstand these loads can add excessive weight to the aircraft, thereby overcoming any advantages of a high aspect ratio wing design.
In the early days of aircraft development, when engines provided a relatively poor power-to-weight ratio, many aircraft were designed with multiple aerofoils, and a biplane design was common. An advantage of a biplane design is that the two aerofoils can be connected together by a combination of struts and bracing wires, so as to form an integrated structure. In effect, the two wings can form a composite beam having a depth equal to the spacing between the wings, hence a relatively light structure can provide significant structural strength. Commonly, in biplane or other multi-aerofoil designs, each wing or aerofoil would have generally similar or comparable dimension and they would all contribute generally equally to the lift. Additionally, each of the wings or aerofoils would have generally similar design characteristics in terms of aspect ratio, aerofoil section, etc.
As aircraft developed and engines became more powerful and reliable, aircraft speeds increased and it was possible to provide sufficient lift from just one wing. Hence, for at least the last sixty years, a monoplane design has been the most popular configuration for most practical aircraft. It is commonly believed that a monoplane design provides the most efficient aerofoil and, by eliminating any extra aerofoils, can reduce the overall drag characteristics of an aircraft.
In early aircraft, wings extended essentially transversely with little or no sweepback. Modern, high-speed aircraft, typically cruise at a speed relatively close to the speed of sound, for example, at Mach 0.8. At such speeds, it is necessary to provide wings with significant sweepback, in order to reduce drag.
Considerable effort and analysis is put into designing aircraft structures and particularly aircraft wings for modern aircraft. Commonly, sophisticated computational techniques are used to develop structures providing the highest degree of aerodynamic efficiency, while also being structurally efficient. As a result, many modern turbofan powered airline aircraft have sweptback, low aspect ratio wings. These wings commonly show a wing chord that is largest at the root and decreases significantly along the length of the wing towards the wing tip, so that the chord ratio between the wing root and the wing tip can be as high as 5. The overall wing aspect ratio can be less than 8.
Now, aircraft drag is directly related to the weight of an aircraft, which in turn will depend upon the weight of fuel carried by the aircraft. Fuel represents a significant element of a weight of an aircraft, and clearly the weight of the fuel will depend upon the intended duration of a flight. Fuel also represents one of the significant items in the operating cost of an airline company. The world""s largest airline companies typically spend from $1 billion to $2 billion, each per year for fuel (Handbook of Airline Economics, First Edition, page 367, published by the Aviation Week Group). Thus, if a large airline company would purchase the aircraft that includes the 29.1% drag reduction, it would save between $291 million to $582 million per year. It will be shown later that a drag reduction of this order should be achievable for a typical modern aircraft. This would lead to a significant increase in net profit.
What the present inventor has realized is, despite all the effort put into designing modern aircraft wings, the low aspect ratio wings of many modern aircraft generate significant induced drag. The inventor has realized that this can be alleviated by providing a drag reduction system (DRS). This is counterintuitive, since it both increases the weight of the aircraft and increases drag, other than induced drag. However, it will be shown that if the supplementary wing of the DRS is given a high aspect ratio, it will cause a large reduction of both the induced drag and the profile drag of the wing of a modern high speed high altitude turbofan powered aircraft with a low aspect ratio wing, such as a Boeing 757-300. This reduction in drag will be much greater than any additional drag added by the DRS, thereby leading to an overall reduction in drag and the possibility of large fuel savings. These fuel savings translate into overall reductions in the operating weight of the aircraft, which more than compensates for the additional weight of the DRS.
It will be appreciated that a number of parameters in the basic aircraft design can be varied considerably. In general, the invention is applicable to high speed, high altitude aircraft, intended for both commercial and military applications. Typical parameters applicable to such aircraft are: a cruising speed in the range Mach 0.6 to 0.9; cruising altitude limits between 30,000 and 50,000 feet; a main wing aspect ratio in the range 7-11; in accordance with the present invention, an aspect ratio for the supplementary wing in the range 15-30; both the main wing and the supplementary wing can have sweep angles in the range 20-50xc2x0, and note that the wings can be swept both forward as well as backward. The invention is believed particularly applicable to aircraft in the weight range 50,000-1,000,000 lbs. Lift limits on the main wing are based on the maximum take-off weight divided by the wing area, and commonly limits are in the range 110-160 lbs per sq ft; supplementary wing area is usually expected to be in the range of 60-90% of the main wing area.
The example below gives a calculated drag reduction of 29.1%, for a Boeing 757-300 aircraft. This is a medium range airliner. It is expected that application of this invention to a low range aircraft would yield a substantially lower percentage drag reduction; on the other hand, application to a high range aircraft should give a substantially higher percentage drag reduction.
In accordance with a first aspect of the present invention, there is provided an aircraft comprising: a body; a supplementary wing comprising two wing sections extending outwardly on either side of the body, to provide lift for the body; and a wing support structure for the supplementary wing extending between the body and the supplementary wing, wherein the supplementary wing is configured to reduce induced drag and the supplementary wing by itself has insufficient strength to support the aircraft, wherein the wing support structure comprises a main wing and provides additional strength to withstand loads applied to the supplementary wing by lift forces at least and wherein the main wing extends outwardly from the body spaced from the supplementary wing and the wing support structure includes at least two connections between the main wing and the supplementary wing transferring loads between the supplementary wing and the main wing.
Preferably, each wing section of the supplementary wing comprises a supported inner section and a cantilevered section extending out from the supported inner section, the supported inner section being connected to the main wing by the connections. The supported inner section of each wing can extend over more than half the length of each wing section.
The connections advantageously comprise struts that are in tension between the supplementary wing and the main wing.
The main wing is preferably adapted to provide part of the lift required for the aircraft during take-off and landing, and includes at least one of: high lift devices; ailerons; an air brake; and a fuel storage means.
The supplementary wing can be such as to provide between 65% to 85% of the total lift of the aircraft in cruising flight at least.
Another aspect of the present invention provides a method of reducing the drag of an existing high speed aircraft, which comprises a body and an existing wing structure having a low aspect ratio wing, the method comprising:
(a) providing a supplementary wing comprising a pair of supplementary wing sections and having a higher aspect ratio than the existing wing but insufficient strength to transfer the full lift required to the aircraft fuselage, whereby the supplementary wing has a relatively light weight.
(b) attaching the supplementary wing to the body and the existing wing of the aircraft and configuring the supplementary wing so that a major portion of the lift required for the aircraft is provided by the supplementary wing, and so that a substantial part of the lift provided by the supplementary wing is transferred to the existing wing.
Many modern aircraft have low mount wings, and for such aircraft, the method can comprise mounting the supplementary wing to the top of the body and spaced above the existing wing.